The emulsion polymerization process enjoys wide usage in the chemical industry. Many monomers can be polymerized using this technology. The method combines the economy and safety of an aqueous reaction medium with a rapid but readily controlled polymerization to provide high molecular weight polymers in excellent yield. Among the advantages of emulsion polymerization are reduced fire hazard, high reaction rate, production of high molecular weight polymer relative simplicity of the technology and production of low viscosity latexes. Shortcomings of emulsion polymerizations include the relative difficulty in separating the polymer product and contamination of the polymer product with residual emulsifier.
Chain-transfer agents can be and are added to emulsion polymerization systems in order to improve final polymer properties. Chain-transfer agents are generally used in free radical polymerizations and function as molecular weight modifiers. The chain-transfer agent reacts with a growing polymer chain to form a "dead" polymer with the concurrent formation of a new center for polymer growth. The chain transfer mechanism for mercaptan containing chain-transfer agents can be depicted as follows: ##STR1##
Among those polymer properties which it is desirable to control are polymer molecular weight distribution, freeze-thaw stability of the latex and polymer particle size. A narrow molecular weight distribution refers to a more nearly homogeneous polymer sample than a similar polymer blend having a broad molecular weight distribution. For many applications, polymers having a narrow molecular weight distribution is desirable.
Freeze-thaw stability refers to the ability of a latex to remain as an emulsion even when exposed to heat-cool temperature cycles. Since emulsion latexes produced from acrylic monomers are susceptible to breakdown of the emulsion when exposed to temperature extremes over a period of time, emulsion latexes which are stable to exposure to freeze-thaw conditions are desirable in that breakdown of the emulsion with resultant settling of the polymer would be avoided.
For certain applications, such as for example in coatings and adhesives, a polymer latex having a relatively uniform particle size would be desirable. It is generally observed, however, that acrylic polymers produced by emulsion polymerization have a rather broad particle size distribution, and thus are not well suited for the production of homogeneous coatings or the like.
It is a continuing goal of those in the emulsion polymerization art to discover new chain-transfer agents which are effective for molecular weight modification and which provide polymer products with improved physical properties.